1. Field of the Invention
The present invention relates to a flame or flameless atomic absorption spectrophotometer, and more particularly, it relates to an atomic absorption spectrophotometer which performs background correction by a self-inversion system with a hollow cathode lamp employed as a light source.
2. Description of the Background Art
In a hollow cathode lamp, a hollow cylindrical cathode of a measured element and an anode of Ni or the like are loaded in a glass tube with a low-pressure filler gas, and a voltage is applied across the anode and the cathode so that free electrons collide with atoms of the filler gas to generate gas ions, which in turn are accelerated to collide with the cathode, thereby sputtering the cathode. The as-sputtered cathode atoms are excited through further collision with the gas ions, to emit a light of a characteristic spectrum of the cathode material.
Even if a lighting current is increased in order to obtain intense light in such a hollow cathode lamp, the light intensity is not necessarily increased. When the lighting current is increased, a large quantity of neutral atoms are generated in the cathode part to cause self- absorption of a resonance line. To this end, known is a hollow cathode lamp which is provided with a third electrode (boost electrode) to discharge neutral atoms thereby preventing self-absorption and increasing light intensity. Such a hollow cathode lamp provided with a boost electrode is described in U.S. Pat. No. 4,885,504, for example. The present invention utilizes such a hollow cathode lamp provided with a boost electrode as a light source.
In general, a hollow cathode lamp provided with a boost electrode is so employed that a current is regularly fed to the boost electrode to extract emissive light having strong light intensity. It is possible to perform spectroanalysis with an excellent S-N ratio, by increasing the emissive light in intensity and extracting a luminescent line having small self-absorption.
In an atomic absorption spectrophotometer employing a hollow cathode lamp provided with no boost electrode, a self-inversion method is carried out as one of background correction methods. In such a self-inversion method, absorbance measured with light which is emitted when a large current causing self-absorption is fed across an anode and a cathode is subtracted from that measured with light which is emitted when a small current is fed across the same, to carry out background correction. In this case, however, the result of measurement has an inferior S-N ratio due to weak radiant intensity for measuring atomic absorption.
An atomic absorption spectrophotometer employing such a self-inversion method is described in U.S. Pat. No. 4,462,685, for example.